Pityophthorus juglandis
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Author(s): Javier Mercado, Colorado State University
Source: IPM Images
Pityophthorus juglandis Blackman , 1928 - (walnut twig beetle)
This species feeds on the phloem of walnut trees and the feeding sites then become infected by Geosmithia morbida, a symbiotic fungus carried by the beetle. The attacks result in the development of "thousand cankers", a progressive, debilitating and often fatal disease. The beetle is native to parts of south-western North America where it attacks Arizona walnut, Juglans major, and is not considered a significant pest. It has expanded its range in the western U.S.A. and since 2010 has been also recorded from several new areas in the southern and eastern U.S.A. P. juglandis is also present in Mexico, but does not carry G. morbida there. Since 2013, both the beetle vector and the fungus have been recorded from Italy.
The beetle attacks the stem and larger branches of several species of Juglans, in particular eastern black walnut (Juglans nigra). It constructs galleries in the phloem and small, black cankers develop around the galleries. These are not visible unless the bark is removed and a large number of beetle attacks are necessary before the tree shows external signs of decline. The fungus does not move systemically within the tree. Mass attacks by the beetle are initiated by an aggregation pheromone. It attracts both sexes and can be used for monitoring the beetle.
The development from egg to adult lasts around 1½-2 months and there may be 2 or more overlapping generations per year. Adults overwinter under the bark.
- Other images of Pityophthorus juglandis (IPM Images - click to enlarge)
adults on 1 cent coin
male head
posterior end
cankers of Geosmithia morbida and beetle galleries
infestation on walnut
galleries on walnut trunk
pheromone trap (Lindgren funnel)
